Universal procedure for refolding recombinant proteins

ABSTRACT

A universal folding method that has been demonstrated to be effective in refolding a variety of very different proteins expressed in bacteria as inclusion bodies has been developed. Representative proteins that can be dissolved and refolded in biologically active form, with the native structure, are shown in Table I. The method has two key steps to unfold and then refold the proteins expressed in the inclusion bodies. The first step is to raise the pH of the protein solution in the presence of denaturing agents to pH greater than 9, preferably 10. The protein solution may be maintained at the elevated pH for a period of up to about 24 hours, or the pH immediately decreased slowly, in increments of about 0.2 pH units/24 hours, until the solution reaches a pH of about 8.0, or both steps used. In the preferred embodiment, purified inclusion bodies are dissolved in 8 M urea, 0.1 M Tris, 1 mM glycine, 1 mM EDTA, 10 mM beta-mercaptoethanol, 10 mM dithiothreitol (DTT), 1 mM redued glutathion (GSH), 0.1 mM oxidized glutathion (GSSG), pH 10. The absorbance at 280 nm (OD280) of the protein solution is 5.0. This solution is rapidly diluted into 20 volumes of 20 mM Tris base. The resulting solutin is adjusted to pH 9.0 with 1 M HCl and is kept at 4° C. for 24 hr. The pH is adjusted to pH 8.8 and the solution is kept at 4° C. for another 24 hrs. This process is repeated until the pH is adjusted to 8.0. After 24 hr at pH 8.0, the refolded proteins can be concentrated by ultrafiltration and applied to a gel filtration column for purification.

BACKGROUND OF THE INVENTION

[0001] The present invention is generally in the field of methods formanufacture of recombinant proteins, and especially in the field ofrefolding of recombinant proteins expressed in the inclusion bodies ofprocaryotic expression systems such as E. coli.

[0002] This application claims priority to U.S. Ser. No. 60/ 177,836filed Jan. 25, 2000 by Lin, et al., U.S. Ser. No. 60/178,368 filed Jan.27, 2000 by Lin, et al., and U.S. Ser. No. 60/210,292 filed Jun. 8, 2000by Hong, et al., and to U.S. Ser. No. 60/210,306 filed Jun. 8, 2000 byLin.

[0003] Expression of recombinant proteins with natural biologicalactivity and structure, referred to as “proteomics”, becomesincreasingly important with the completion of genomic sequencing forseveral organisms and the near completion of human genome sequencing.One aspect of proteomics is to express large amounts of protein forstructural and functional studies, as well as for commercialapplications. The least expensive and most efficient way to expressrecombinant proteins is to express the proteins in E. coli. Proteins areexpressed either intracellularly or secreted into the periplasmicspaces. In the former case, the proteins are often deposited ininclusion bodies, especially if the protein has disulfide bonds.

[0004] However, one of the problems in expressing mammalian proteins inE. coli is that most of the expressed proteins form insoluble inclusionbodies. While this problem can be circumvented by using variousmammalian or insect expression systems, growing E. coli is faster andless expensive compared to mammalian and insect cultures. Moreover, someproteins are toxic to the host when expressed in their native forms,thus expression as insoluble inclusion bodies is the only way to obtainlarge quantities of recombinant proteins. Importantly, high levels ofexpression can be achieved for most proteins. 400 to 600 mg of inclusionbodies per liter of bacterial culture can routinely be achieved, with upto 9,700 mg/L having been reported using this method (Jeong KL; Lee SY,1999. Appl. Environ. Microbiol. 65:3027-32). Inclusion bodies can beeasily purified to greater than 90% with a simple freeze/thaw anddetergent washing procedure.

[0005] Inclusion bodies appear as dense cytoplasmic granules when thecells are observed under a light microscope. Typically, the cells willbe lysed by mechanical disruption of the cells, followed bycentrifugation for 30 min at 4700 g. Inclusion bodies will sediment atlow g forces and can be separated from many other intracellularproteins. Further purification can be done by washing the pellet withthe buffer used during the cell disruption, or by centrifuging theresuspended pellet in 40-50% glycerol.

[0006] Many extracellular proteins of eukaryotes contain disulfidebonds. Proteins having multiple disulfide bonds may form non-nativedisulfide bonds during folding from the reduced species. Further foldingis then blocked unless the incorrect disulfide bond is cleaved byreduction with an external thiol or by attack from a protein thiol.Eukaryotic organisms that secrete disulfide containing proteins alsomachinery for ensuring proper disulfide bond formation. A distinctdisadvantage of expression of recombinant proteins in prokaryotes asinclusion bodies is that the proteins are not obtained in their nativestate, and typically are not functionally active. A variety of methodshave been used to re-solubilize the proteins and refold them to reformactive protein. Dissolution of the pelleted recombinant protein usuallyrequires the use of denaturants such as 7 M guanidine hydrochloride or 8M urea. The amount of aggregation may continue to increase with time ifthe protein is allowed to remain in the denaturant (Kelley and Winkler,“Folding of Eukaryotic Proteins Produced in Escherichia coli” GeneticEngineering 12, 1-19 at p. 6 (1990)). Removal of the denaturant from thesolubilized inclusion bodies by dialysis or desalting columns will causethe protein to precipitate under conditions where the native proteinneeds to be refolded. A misfolded protein solution can also have a verylow specific activity in biological assays.

[0007] Although there are many reports of expression and refolding ofvarious proteins in E. coli as inclusion bodies, one of themisconceptions in protein refolding is that a unique refolding methodhas to be developed for each individual protein (see Kelley and Winklerat p. 6). Another misconception is that most of the mammalian proteinscannot be refolded from inclusion bodies. (for review, see: Rudolph R.,Lilie H., 1996. FASEB J 10:49-56; Lilie H, Schwarz E, Rudolph R. 1998.Curr Opin Biotechnol 9:497-501). Because published works are mostly“success” stories in refolding inclusion bodies from E. coli, it isimpossible to get a general idea about what percentage of mammalianproteins can be purified using this procedure.

[0008] There are probably more refolding methods than refolded proteinsreported in the literature (for review, see: Rudolph R., Lilie, H. 1996,FASEB J 10:49-56; Lilie, H., Schwarz, E., Rudolph, R. 1998, Curr. Opin.Biotechnol. 9:497-501). Different chaperones, detergents, and chaotrophshave been used to help refolding. In addition, pH, ionic strength,temperature, buffer formulation, and reducing/oxidation reagents can alleffect refolding. It would be prohibitive to test all these conditionsfor refolding large amounts of proteins, as required for studies inproteomics or structural genomics.

[0009] A single simplified procedure to refold most of the proteins thatare expressed in recombinant systems, especially those which forminclusion bodies in systems such as E. coli, is therefore needed.

[0010] It is therefore an object of the present invention to provide a“universal” method for refolding of proteins, especially recombinantproteins, especially recombinant proteins present in inclusion bodies inbacterial hosts.

SUMMARY OF THE INVENTION

[0011] A universal folding method that has been demonstrated to beeffective in refolding a variety of very different proteins expressed inbacteria as inclusion bodies has been developed. Representative proteinsthat can be dissolved and refolded in biologically active form, with thenative structure, are shown in Table I. The method has two key steps tounfold and then refold the proteins expressed in the inclusion bodies.The first step is to raise the pH of the protein solution in thepresence of denaturing agents to pH greater than 9, preferably 10. Theprotein solution may be maintained at the elevated pH for a period of upto about 24 hours, or the pH immediately decreased slowly, in incrementsof about 0.2 pH units/24 hours, until the solution reaches a pH of about8.0, or both steps used. In the preferred embodiment, purified inclusionbodies are dissolved in 8 M urea, 0.1 M Tris, 1 mM glycine, 1 mM EDTA,10 mM beta-mercaptoethanol, 10 mM dithiothreitol (DTT), 1 mM reduedglutathion (GSH), 0.1 mM oxidized glutathion (GSSG), pH 10. Theabsorbance at 280 nm (OD280) of the protein solution is 5.0. Thissolution is rapidly diluted into 20 volumes of 20 mM Tris base. Theresulting solutin is adjusted to pH 9.0 with 1 M HCl and is kept at 4°C. for 24 hr. The pH is adjusted to pH 8.8 and the solution is kept at4° C. for another 24 hrs. This process is repeated until the pH isadjusted to 8.0. After 24 hr at pH 8.0, the refolded proteins can beconcentrated by ultrafiltration and applied to a gel filtration columnfor purification.

DETAILED DESCRIPTION OF THE INVENTION Expression of Recombinant Proteins

[0012] Recombinant proteins are typically expressed in a suitable host,for example, a procaryotic expression system such as E. coli or othertype of bacteria, using a standard expression vector like a plasmid,bacteriophage or even naked DNA, and the protein expressed from theplasmid or DNA integrated the host chromosome. Suitable bacterialstrains are commercially available or can be obtained from the AmericanType Culture Collection, Rockville, Md. (the “ATCC”).

[0013] Suitable vectors can be obtained from any number of sources,including the ATCC. These need a promoter to insure that the DNA isexpressed in the host, and may include other regulatory sequences. Thevector may also include means for detection, such as an antibioticresistance marker, green fluorescent protein tag, or antigen tag tofacilitate in purification of the recombinant protein.

[0014] Once the DNA encoding the protein to be purified is introducedinto the host, the host is cultured under appropriate conditions untilsufficient amounts of recombinant protein are obtained.

Refolding/Purification Methods

[0015] Once the protein has been expressed in the maximum amount, itmust be separated and purified from the bacterial host. The protein isisolated generally by lysing the cells, for example, by suspending indetergent, adding lysozyme, and then freezing (for example, bysuspending cells in 20 ml of TN/1% Triton™ X-100, adding 10 mg lysozymeand freezing at −20° C. overnight), thawing and adding DNAase to degradeall of the bacterial DNA, then washing the resulting precipitate in abuffered solution. The precipitate is then dissolved in an appropriatesolution as discussed below, for refolding.

[0016] The isolated protein is then refolded. There are several criticalaspects of an universal refolding method.

[0017] (i) High pH refolding. Most published procedures refold proteinsusing reducing chaotrophs (such as 8 M urea) at a physiological pH,usually pH 7.4 to 8.0. This usually produces large quantities ofprecipitation or aggregation, making refolding either impossible or witha very low yield. It has been found that some proteins cannot berefolded at physiological pH, but can be refolded when initial refoldingpH is high (at least pH 9.0, although higher pH, such as pH 10, may bedesirable). This strategy was initially inspired by the fact thatpepsinogen can be reversibly denatured/renatured between pH 8.0 and 9.0.It is postulated that at high pH (such as pH 9.0), proteins can obtainsome secondary structures, allowing it to be refolded more efficientlywhen acidity of the refolding solution is lowered to the biological pH.Later it was found that even for the proteins that could be refolded ata physiological pH, high pH refolding resulted in a better yield. Inaddition, high pH refolding is an excellent method for preventinginitial large scale precipitation.

[0018] (ii) Non-denaturing chaotroph concentration. It has been shown inseveral labs that non-denaturing concentrations of chaotrophic reagents,such as 0.5 to 1.0 M of urea, guanidine hydrochloride, and L-arginine,can be used to assist refolding and stabilize refolded proteins (Rudolphet al. 1996, FASEB J 10:49-56). The preferred concentration ofchaotrophic reagents is 0.4 M, although the concentration may range from0 to 4 M. A moderate concentration of urea in the refolding/purificationprocedures has not been found to have a denaturing effect on proteins.

[0019] (iii) Reducing/oxidation reagents. Inclusion bodies for mammalianproteins containing disfulfide bonds need to be dissolved in thepresence of reducing reagents. Representative reducing agents includebeta-mercaptoethanol, in a range of from 0.1 mM to 100 mM, preferably 10mM; DTT, in a range of from 0.1 mM to 10 mM, preferably 10 mM; reducedglutathion (GSH), in a range from 0.1 mM to 10 mM, preferably 1 mM; andoxidized glutathion (GSSG), in a range from 0.1 mM to 10 mM, preferably1 mM. Beta-mercaptoethanol is a preferred reducing reagent. In addition,dithiothreitol and/or reduced/oxidized glutathion (GSH, GSSG) can alsobe included to facilitate “oxido-shuffling” of wrongly folded,intermediate disulfide bonds.

[0020] (iv) pH control It is important that the protein solution remainat an elevated condition long enough to refold the protein. This ispreferably achieved by decreasing the pH slowly, in 0.2 pH unitincrements per 24 hours. In this method, the protein solution isadjusted to a high pH, preferably at least 9.0 or higher, to 10 or lesspreferably 11. The protein is preferably maintained at each pH for atleast 24 hours, although comparable effects can be achieved with shorterperiods of time, for example, for a period of three, six, nine, twelve,eighteen or twenty hours, most preferably at least twelve hours. The pHcan be adjusted by addition of an acid or by dialysis or dilution into alower pH. Addition of the acid is preferred.

[0021] The four conditions discussed above are considered the mostessential aspects of the basic protocol for the “universal” refoldingprocedure. TABLE I Expression, refolding, and purification of differentproteins from E. coli Purifi- Name From Organism Refold cation Ref.Pepsinogen Full-length Porcine Yes Yes Lin, et al, 1989 Pepsinogen N andC Porcine Yes Yes Lin, et al, 1992 Domain Lin, et al, 1993 Rhizopus-full-length Fungus Yes Yes Chen, et al, 1991 Pepsinogen Lin, et al, 1992Thermopsin full-length Archae No No None Thermopsin fusion Archae YesPartial Lin, Liu, Tang, 92 Cathepsin D full-length human No No None lowyield Pregnancy full-length Bovine No No None Specific Ant Ovine HIVprotease full-length HIV Yes Yes Lin, et al, 1995 Ermolief, et al, 97SAP full-length Yeast Yes Yes Lin, et al, 1993 Koelsch, et al, 98Streptokinase full-length bacteria Yes Yes Wang, et al, 1998 Plasminogencat-domain human Yes Yes Wang, et al, 2000 Cadosin A full-length plantYes Yes Faro, et al, 1999 Napsin 1 full-length human No No Koelsch, etal, 00 Memapsin 2 full-length human Yes Yes Lin, et al, 2000 Memapsin 1full-length human Yes Yes PreS partial HBV Yes Yes unc-76 full-length C.elegans Yes Yes odc-1 full-length C. elegans Yes Yes ceh-10 full-lengthC. elegans Yes Yes ppp-1 full-length C. elegans No No

References for Table 1

[0022] Lin, X, Wong, R. N. S., and Tang, J. (1989) “Synthesis,purification, and active site mutagenesis of recombinant porcinepepsinogen”. J. Biol. Chem. 264:4482-4489.

[0023] Lin, X. L., Lin, Y.-Z., Koelsch, G., Gustchina, A., Wlodawer, A.,and Tang, J. (1992) “Enzymic activities of two-chain pepsinogen,two-chain pepsin, and the amino-terminal lobe of pepsinogen”. J. Biol.Chem. 267:17257-17263.

[0024] Lin, X., Loy, J. A., Sussman, F., and Tang, J. (1993)“Conformational instability of the N- and C-terminal lobes of porcinepepsin in neutral and alkaline solutions”. Prol. Sci. 2:1383-1390.

[0025] Chen, Z., Koelscb, G., Han, H.-P., Wang, X.-J., Lin, X.,Hartsuck, J. A., and Tang, J. (1991) “Recombinant rhizopuspepsinogen”,J. Biol. Chem. 266:11718-11725.

[0026] Lin, Y.-Z., Fusek, M., Lin, X. L., Hartsuck, J. A., Kezdy, F. J.,and Tang, J. (1992) “pH Dependence of kineticparameters of pepsin,rhizopuspepsin, and their active-site hydrogen bond mutants”. J. Biol.Chem. 267:18413-18418.

[0027] Lin, X. L., Liu, M. T., and Tang, J. (1992) “Heterologousexpression of thermopsin, a heat stable acid proteinase”. Enzyme Microb.Technol. 14:696-701.

[0028] Lin, Y.-Z., Lin, X., Hong, L., Foundling, S., Heinrikson, R. L.,Thaisrivongs, S., Leelamanit, W., Raterman, D., Shah, M., Dunn, B. M.,and Tang, .J (1995) “Effect of point mutations on the kinetics and theinhibition of human immunodeficiency virus type 1 protease: relationshipto drug resistance”. Biochemistry 34:1143-1152.

[0029] Ermolieff, J., Lin, X., and Tang, J. (1997) Kinetic properties ofsaquinavir-resistant mutants of human immunodeficiency virus type 1protease and their implications in drug resistance in vivo. Biochemistry36:12364-12370.

[0030] Lin, X., Tang, J., Koelsch, G., Monod, M., and Foundling, S.(1993) “Recombinant canditropsin, an extracellular aspartic proteasefrom yeast Candida tropicalis”. J. Biol. Chem. 268:20143-20147.

[0031] Koelsch, G., Tang, J., Monod, M., Foundling, S. I., Lin, X.(1998) “Primary substrate specificities of secreted aspartic proteasesof Candida albicans”. Adv. Exp. Med. Biol. 436:335-338.

[0032] Wang, X., Lin, X., Lowy, J. A., Tang, J., Zhang, X. C. (1998)“Crystal structure of the catalytic domain of human plasmin complexedwith streptokinase”, Science. 281:1662-1665.

[0033] Wang, X., Terzyan, S., Tang, J., Loy, J., Lin, X., and Zhang, X.(2000) “Human plasminogen catalytic domain undergoes a novelconformational change upon activation” J. Mol. Biol. (in press).

[0034] Faro, C., Ramalho-Santos, M., Vieira, M., Mendes, A., Simoes, I.,Andrade, R., Verissimo, P., Lin, X., Tang, J., Pires, E. (1999) “Cloningand Characterization of cDNA Encoding Cardosin A, an RGD-containingPlant Aspartic Proteinase,” J. Biol. Chem. 274(40):28724-28729.

[0035] Lin, X., Koelsch, G., Wu, S., Downs, D., Dashti, A., and Tang, J.(2000) “Human aspartic protease memapsin 2 cleaves the β-secretase siteof β-amyloid precursor protein. Proc. Natl Aca. Sci. 97(4):1456-1460.

[0036] Ghosh, A. K., Shin, D., Downs, D., Koelsch, G., Lin, X.,Ermolieff, J., Tang, J. (2000) “Design of potent inhibitors form humanbrain memapsin 2 (βsecretase)” J. Amer. Chem. Soc., 122:3522-3523.

[0037] The present invention will be further understood by reference tothe following non-limiting examples.

Example 1 Preferred Method for Refolding Recombinant Proteins

[0038] A. Reagents:

[0039] ZB media: 10 g N-Z-Amine A, 5 g NaCl/L

[0040] LB media: 10 g Tryptone, 5 g Yeast extract, 10 g NaCl/L, pH 7.5

[0041] 8 M urea: 8 M urea, 0.1 M TRIS™, 1 mM glycine, 1 mM EDTA, pH 10

[0042] TN buffer: 0.05 mM TRISTM, 0.15 M NaCl, pH 7.5

[0043] B. Expression of the Recombinant Protein:

[0044] 1. Expression plasmids should be transfected into an appropriatehost, such as the BL21(DE3) strain of E. coli and plated onZB/Ampicillin plates, which selects for the desired recombinantorganisms. A single colony from each construct is inoculated into 100 mlof ZB/ampicillin media and grown 16 h at 37° C.

[0045] 2. Inoculate 20 ml of the overnight culture into 1 L ofLB/ampicillin, and shake at 37° C. till OD₆₀₀ reaches 0.4-0.6. Add IPTGto 0.5 mM, then shake for 3 h.

[0046] 3. Centrifuge and resuspend cells in 20 ml of TN/1% Triton™X-100. Add 10 mg lysozyme and freeze at −20° C. overnight.

[0047] 4. Thaw the frozen cells, add 20 μl 1 M MgSO₄ 100 μg DNAase, andstir until the bacterial DNA is completely dissolved.

[0048] 5. Add 250 ml of TN/1% Triton and stir for 2-4h. Centrifuge andrepeat the Triton wash one more time.

[0049] 6. Dissolve the pellet in 10 ml of 8 M urea solution, addbeta-Mercaptoethanol to 100 mM. This solution can be ultracentrifuged,and is then ready for refolding.

[0050] C. Refolding and Purification:

[0051] The OD₂₈₀ of the solution containing the inclusion bodies isadjusted to 5.0 with the 8 M urea solution. The final solution containsthe following reducing reagents:

[0052] 10 mM beta-Mercaptoethanol

[0053] 10 mM DTT (Dithiothreitol)

[0054] 1 mM reduced glutathion (GSH)

[0055] 0.1 mM oxidized glutathion (GSSG)

[0056] The final pH of the solution is 10.0.

[0057] 1. The above solution is rapidly diluted into 20 volumes of 20 mMTRIS™ base, the pH is adjusted to 9.0, and then slowly adjusted to 8.0with 1 M HCl, by adjusting pH to 8.8 for twenty four hours, then 8.6 fortwenty four hours, etc., until the pH is 8.0. Alternatively, theproteins can be refolded using dialysis. The OD₂₈₀ of the 8 M ureasolution is adjusted to 0.5, and dialyzed against 20 volumes of TRIS™base. The pH of the solution is again slowly adjusted to 8.0.

[0058] 2. The refolded material is then concentrated by ultrafiltration,and separated by gel filtration, for example, on a SEPHACRYL™ S-300column equilibrated with 20 mM TRIS™, HCl, 0.4 M urea, pH 8.0.

[0059]3. The S-300 fractions can be checked by running a non-reducedSDS-PAGE. The wrongly refolded protein runs at a very high molecularweight, while folded proteins run at a normal molecular weight.

[0060] 4. The refolded peak from the S-300 column can be furtherpurified with a FPLC Resource-Q™ or Resource-S™ column, which isequilibrated with 20 mM TRIS™-HCl (HEPES buffer for Resource-S™), 0.4 Murea, pH 8.0. The enzyme is eluted from the column with a lineargradient of NaCl. Table I lists all of the proteins which have beenrefolded directly or as inclusion bodies.

EXAMPLE 2. Expression and Refolding of Memapsin 2

[0061] Pro-memapsin 2 was PCR amplified and cloned into the BamHI siteof a pET11 a vector. The resulting vector expresses pro-memapsin 2having a sequence from Ala-8p to Ala 326. Two expression vectors,pET11-memapsin 2-T1 (hereafter T1) and pET11-memapsin 2-T2 (hereafterT2), were constructed. In both vectors, the N-terminal 15 residues ofthe expressed recombinant proteins are derived from the expressionvector. Pro-memapsin 2 residues start at residue Ala-16. The tworecombinant pro-memapsin 2s have different C-terminal lengths. Clone T1ends at Thr- 454 and clone T2 ends at Ala-419. The T1 construct containsa C-terminal extension from the T2 construct but does not express any ofthe predicted transmembrane domain.

[0062] The T1 and T2 expression vectors were separately transfected intoE. coli strain BL21(DE3). The procedures for the culture of transfectedbacteria, induction for synthesis of recombinant proteins and therecovery and washing of inclusion bodies containing recombinant proteinsare essentially as described by Lin et al., 1994 Methods in Enzymology214, 195-224. Basically, the inclusion bodies are washed with 1 % (v/v)Triton X-100 and 0.15 M NaCl in 0.1 M Tris™ HCl, pH 7.4, the insolubleprotein is dissolved in a solution containing 8 M urea, 0.05 Mcyclohexylaminopropanesulfonic acid, 10 mM 2-mercaptoethanol, 10 mM DTT(Dithiothreitol), 1 mM reduced glutathion (GSH), 0.1 mM oxidizedglutathion (GSSG),1 mM glycine, and 1 mM ethylenediaminetetraacetic acid(EDTA), pH 10.5, to a protein concentration of about 5 mg/ml. Thissolution is added dropwise to 20 vol of rapidly stirred 20 mM Tris base.The pH of the diluted solution is readjusted to 9.0 with 1 M HCl andkept at 4 C. for 24 hr. The pH is than adjusted to 8.8 with 1 M HCl, andkept at 4 C. for 24 hr again. The process is repeated until the pH isadjusted to 8.0.

[0063] Three different refolding methods have produced satisfactoryresults.

[0064] (i) The rapid dilution method.

[0065]

[0066] Pro-memapsin 2 in 8 M urea/10 mM DTT (Dithiothreitol), 1 mMreduced glutathion (GSH), 0.1 mM oxidized glutathion (GSSG), withOD_(280nm)−5 was rapidly diluted into 20 volumes of 20 mM-Tris base. Thesolution was slowly adjusted into pH 8 with 1 M HCl. The refoldingsolution was then kept at 4° C. for 24 to 48 hours before proceedingwith purification.

[0067] (ii) The reverse dialysis method

[0068] An equal volume of 20 mM TRIS™, 0.5 mM oxidized/1.25 mM reducedglutathione, pH 9.0 is added to rapidly stirred pro-memapsin 2 in 8 Murea/10 mM beta-mercaptoethanol with OD_(280nm)=5. The process isrepeated three more times with 1 hour intervals. The resulting solutionis then dialyzed against sufficient volume of 20 mM TRIS™ base so thatthe final urea concentration is 0.4 M. The pH of the solution is thenslowly adjusted to 8.0 with 1 M HCl.

[0069] iii. The preferred method for refolding.

[0070] Inclusion bodies are dissolved in 8 M urea, 0.1 M TRIS™, 1 mMGlycine, 1 mM EDTA, 100 mM beta-mercaptoethanol, pH 10.0. The OD₂₈₀ ofthe inclusion bodies are adjusted to 5.0 with the 8 M urea solutionwithout beta-mercaptoethanol. The final solution contains the followingreducing reagents: 10 mM beta-mercaptoethanol, 10 mM DTT(Dithiothreitol), 1 mM reduced glutathion, and 0.1 M oxidizedglutathion. The final pH of the solution is 10.0.

[0071] The above solution is rapidly diluted into 20 volumes of 20 mMTRIS™ base, the pH is adjusted to 9.0, and the resulting solution iskept at 4° C. for 16 hr. The solution is equilibrated to roomtemperature in 6 hr, and the pH is adjusted to 8.5. The solution isreturned to 4° C. again for 18 hr.

[0072] The solution is again equilibrated to room temperature in 6 hr,and the pH is adjusted to 8.0. The solution is returned to 4° C. againfor 4 to 7 days.

Purification of Recombinant Pro-memapsin 2-T1

[0073] The refolded material is concentrated by ultrafiltration, andseparated on a SEPHACRYL™ S-300 column equilibrated with 20 mM TRIS™HCl, 0.4 M urea, pH 8.0. The refolded peak (second peak) from the S-300column can be further purified with a FPLC RESOURCE-Q™ column, which isequilibrated with 20 mM TRISTM-HCl, 0.4 M urea, pH 8.0. The enzyme iseluted from the column with a linear gradient of NaCl. The refolded peakfrom S-300 can also be activated before further purification. Foractivation, the fractions are mixed with equal volume 0.2 M SodiumAcetate, 70% glycerol, pH 4.0. The mixture is incubated at 22° C. for 18hr, and then dialyzed twice against 20 volumes of 20 mM Bis-TRIS™, 0.4 Murea, pH 6.0. The dialyzed materials are then further purified on a FPLCRESOURCE-Q™ column equilibrated with 20 Bis-TRIS™, 0.4 M urea, pH 6.0.The enzyme is eluted with a linear gradient of NaCl.

[0074] Modifications and variation of these methods are intended to comewithin the scope of the appended claims.

I claim:
 1. A method for refolding of recombinant proteins comprisingmaintaining the protein at a pH of 9.0 or greater, in the presence ofone or more chaotrophic and reducing agents, and decreasing the pH ofthe solution gradually over a period of at least about 24 hrs to pH 8.0to induce renaturation of at least a portion of the protein so that itqualitatively exhibits a biological activity and structurecharacteristic of the protein.
 2. The method of claim 1 wherein pH isdecreased in increments equivalent to 0.2 pH units per 24 hours.
 3. Themethod of claim 1 wherein the protein is maintained at a pH of greaterthan 9.0 for a period of at least 24 hours.
 4. The method of claim 1wherein the pH is decreased by addition of acid.
 5. The method of claim1 wherein the pH is decreased by dilution or dialysis into a solution ofa lower pH.
 6. The method of claim 1 wherein the chaotrophic anddenaturing reagents are selected from the group consisting of between0.5 and 1.0 M urea, 0.1 mM to 100 mM beta-mercaptoethanol, 0.1 mM to 100mM DTT, 0.1 mM to 10 mM reduced glutathion, and 0.1 mM to 10 mM oxidizedglutathion.
 7. The method of claim 1 wherein the protein is firstextracted from bacterial inclusion bodies.
 8. The method of claim 4wherein the bacteria is E. coli.
 9. The method of claim 7 wherein theinclusion bodies are dissolved at a final pH of between greater than 9and
 10. 10. The method of claim 1 , wherein the protein is dissolved ata pH above about 10.0.
 11. The method of claim 1 , wherein the proteinis dissolved at a pH above about 11.0.
 12. The method of claim 1 ,wherein the protein is dissolved at a pH above about 12.0.
 13. Themethod of claim 1 wherein the pH is decreased over a period of at leastabout 36 hours.
 14. The method of claim 1 , wherein the pH of thesolution is reduced more quickly at the higher pH and more graduallynearer the physiological pH range of the protein.
 15. The method ofclaim 1 comprising the additional step of separating protein specieswhich exhibits biological activity from inactive, or wrongly folded,protein species.